The present invention relates generally to multi-layer antireflection coatings for substrates, and more particularly to multilayer antireflection coatings for visible or near infrared light deposited on transparent or semitransparent substrates by sputtering.
The simplest antireflection (AR) coating is a single layer of a transparent material having a refractive index n which is less than that of the substrate on which it is deposited. According to J. Strong, "On a Method of Decreasing the Reflection from Nonmetallic Substrates," J.Opt.Soc.Am., Vol. 26, January 1936, pp. 73-74, the optimum index n for the layer is equal to the square root of the index of the substrate. The optical thickness of the layer (n times the actual thickness d of the layer) is typically about one quarter of the central wavelength of the spectral region for which the reflectance is to be reduced. For visible light, this central wavelength is about 510 or 520 nm. Such a single-layer coating produces a minimum reflectance at the central wavelength. At all other nearby wavelengths, the reflectance is higher than the minimum value but lower than the reflectance of the uncoated substrate. Coating a glass substrate with a refractive index of 1.52 with a quarterwave layer of MgF.sub.2 having an index of 1.38 reduces the reflectance of the glass from about 4.26% to about 1.26% at the central wavelength.
Multilayer AR coatings are typically made by depositing two or more layers of transparent dielectric materials on a substrate. One type of multilayer AR coating consists of layers with indices of refraction less than the substrate, with the layers arranged in order of decreasing index of refraction from the substrate outwards. See L. Young, "Antireflection Coatings on Glass," Applied Optics 4, 366-367 (1965) and U.S. Pat. No. 5,262,633. The more common multilayer AR coating includes one or more layers with indices of refraction higher than the index of refraction of the substrate. The simplest such multilayer AR coating consists of two layers. The first layer on the substrate is a high-index layer having an optical thickness less than a quarterwave at the central wavelength. This layer is followed by a low-index layer with an optical thickness greater than a quarterwave at the central wavelength. See U. S. Pat. Nos. 2,281,474 and 2,782,676. Such a coating can generally achieve a zero reflectance at the central wavelength. The disadvantage of this type of coating is that on each side of the central wavelength, the reflectance increases sharply. Thus, if the central wavelength lies within the visible spectrum, the reflectance values at some wavelengths in the visible spectrum are much higher than for the single-layer AR coating and in some cases higher than the uncoated substrate itself. Because of the V shape of the reflectance curve, this type of AR coating is often referred to as the V-Coat design.
A special case of the two-layer AR coating occurs if the high-index layer has a specific value of the index of refraction. Let n.sub.H denote the index of the high-index layer; n.sub.L the index of the low-index layer; and n.sub.S the index of the substrate. Then if n.sub.H.sup.2 =n.sub.S n.sub.1.sup.2, the correct optical thicknesses of the high- and low-index layers to give zero reflectance at the central wavelength are both one quarter of the central wavelength. This design is sometimes referred to as the quarter-quarter (QQ) AR coating. See Musset et al., "Multilayer Anti-reflection Coatings," Progress in Optics 8, pp. 201-237 (1970). For example, if the glass substrate has a refractive index n.sub.S of 1.52 and the low-index outer layer has an index n.sub.L =1.38, then the correct index n.sub.H of the high-index layer for the QQ design is 1.70. One way of describing the action of the QQ design is to say that the high-index first layer raises the effective index of the substrate to a value such that the low-index second layer can provide a perfect AR coating at the central wavelength.
A simple broad-band AR coating consists of three layers. The first layer of the three-layer AR coating deposited on the glass substrate generally has a medium index of refraction, specifically higher than that of the substrate, and an optical thickness which is about one quarter of the central wavelength. The second layer has a high index of refraction, specifically higher than the first layer, and an optical thickness which is about one-half of the central wavelength. The third layer has a low refractive index, specifically lower than the first layer and generally lower than that of the substrate, and an optical thickness which is one quarter of the central wavelength. The three-layer design is described in Gaiser U.S. Pat. No. 2,478,385; Thelen U.S. Pat. No. 3,185,020; and Lockhart et al., "Three-Layered Reflection Reducing Coatings," J.Opt.Soc.Am. 37, pp. 689-694 (1947). This three-layer AR coating is often referred to as the quarter-half-quarter (QHQ) design. One way of describing the action of the QHQ design is to say that the medium-index first layer raises the effective index of the substrate, the high-index second layer provides the broad band performance, and the low-index third layer provides the anti-reflective property.
A disadvantage of the three-layer design is that the refractive indices of the three layers must have specific values in order to produce optimum performance. The selection and control of the refractive index of the first layer is particularly important. Deviation from specific refractive index values cannot be compensated for by varying the thicknesses of the layers.
Various modifications of the three-layer AR coating have been made to overcome this disadvantage. For example, one can form the second layer from a mixture of two materials to achieve the optimum refractive index as disclosed in U.S. Pat. No. 3,604,784. One can replace the second layer by two high-index layers, each having an optical thickness which is a quarter of the central wavelength as disclosed in U.S. Pat. No. 3,463,574. A fourth layer with refractive index lower than that of the substrate and an optical thickness of one-half of the central wavelength can be added between the medium-index layer and the substrate as disclosed in U.S. Pat. No. 3,781,090. A variety of AR coatings involving quarterwave- and halfwave-thick layers of various refractive indices are described in Musset, et al. (1970) and in Baumeister et al., "Application of Linear Programming to Antireflection Coating Design," J.Opt.Soc.Am. 67, pp. 1039-1045 (1977).
Another broad-band AR coating was disclosed by Millendorfer in U.S. Pat. No. 3,235,397. This AR coating consists of four or more alternating layers of two materials, having alternately a high and a low index of refraction. Using this approach, broad-band AR coating performance can be achieved using a range of high-index materials if the low-index material has an index of refraction similar to that of MgF.sub.2 --typically less than about 1.38. Similar AR coatings are discussed in U.S. Pat. Nos. 5,460,888 and 3,761,160.
The AR coatings discussed above are generally deposited by thermal evaporation. Specifically, MgF.sub.2 can only be easily deposited by thermal evaporation. In addition, the times required for depositing layers by thermal evaporation are usually only a small fraction of the total production time. The production of coatings by thermal evaporation may include time to pump down the coating chamber, time to heat the substrates to process temperature, and time to cool the substrates after coating. The number of layers in an AR coating, the thicknesses, and the specific materials may not have a significant impact on the total production time or cost.
Magnetron sputtering is the process most often used for large area commercial coating applications. Magnetron sputtering involves sputtering from targets of selected materials in the close proximity of strong magnets using either a DC or an AC power supply. See U.S. Pat. Nos. 4,166,018 and 4,046,659. Most di-electric materials are sputtered reactively from a metal target using a reactive sputtering gas such as oxygen. Magnetron sputtering may be carried out in an in-line system to deposit thermal control coatings on architectural and automobile glazings and AR coatings on glass or cathode ray tubes (CRTs) used for computer monitors or television sets. In the in-line sputtering system, the articles to be coated pass through an entry lock, and then are passed through a vacuum chamber containing a series of sputtering sources, called cathodes or targets. The terms cathode and target are often used interchangably, but strictly speaking the target is the material being sputtered, while the cathode includes the target, the magnets and electrical connections necessary to enable the sputtering process. After coating, the articles pass through an exit lock.
For an in-line sputtering system, the heating and pumpdown are done concurrently with the coating. Thus, some parts are being heated (if necessary) and some are being pumped down while others are being coated. Thus, the time taken to deposit the layers is an important factor. This time depends on thicknesses of the layers and on the deposition rates of the materials chosen for the coating. If the coating requires a thick layer of a material with a slow deposition rate, either the throughput of the system will be low because of the long time required to coat the layer or the in-line system will be large and therefore expensive because many sputtering cathodes will be necessary to deposit the layer rapidly enough to keep up with the desired throughput.
Many of the materials commonly used in thermal evaporation processes, particularly fluorides and sulfides, are not easily sputtered. In particular, the low-index material MgF.sub.2 cannot practically be deposited in a sputtering system. Additionally, the high-index material TiO.sub.2 has an extremely slow sputter-deposition rate. Thus, while TiO.sub.2 is a desirable material for thermal evaporation, its use in large scale in-line sputtering systems comes with a penalty of low through-put or high coater cost.
A major improvement in the earlier AR coatings was introduced by Rock in U.S. Pat. No. 3,432,225. The Rock AR coating is made from two coating materials, one material having a high index of refraction, generally greater than 2, and the other having a low index of refraction, generally lower than that of the substrate. The Rock AR coating consists of four layers. The first layer adjacent to the substrate is of the high-index material and has an optical thickness which is approximately one-tenth of the central wavelength of the anti-reflection band. The second layer from the substrate is of the low-index material and has an optical thickness which is about one-tenth of the central wavelength. The third layer from the substrate is of the high-index material and has an optical thickness which is about one-half of the central wavelength and the outer layer is of the low-index material and has an optical thickness which is about one-quarter of the central wavelength. The advantage of the Rock AR coating is that materials with specific refractive index values are not required; the thicknesses of the layers can be adjusted to give a low reflectance value across the visible spectrum for a range of possible material indices.
The basis for the design of the Rock AR coating is described in the Rock patent in terms of a construction called the polar-coordinate phase diagram, more commonly called the circle diagram. For a description of the circle diagram, see Apfel, "Graphics in Optical Coating Design," Applied Optics 11, 1303-1321 (1972). An important feature of the Rock AR coating in terms of the circle diagram is that the third layer gives rise to a full circle which lies completely to the left of the circle segment of the final layer. One way of describing the working of the Rock AR coating is to say that the first two layers raise the effective index of the substrate, the halfwave-thick third layer provides the broad band performance, and the final layer provides the antireflective property. This description is similar to that of the three-layer AR, except that the first two layers of the Rock AR coating perform the function of the medium-index first layer of the three-layer AR.
For evaporated coatings, the Rock AR coating is an economical solution to the problem of a broad-band AR coating. It requires only two coating materials, which can be used to deposit AR coatings on substrates with a wide range of refractive indices. The thick high-index layer is not a problem because, as was stated above, for evaporated AR coatings the deposition time is usually a small fraction of the total time necessary to produce a coating. There are several high-index materials which are suitable for the high-index layer deposited by evaporation, such as TiO.sub.2, HfO.sub.2, ZrO.sub.2, Ta.sub.2 O.sub.5, and Nb.sub.2 O.sub.5, and mixtures of these materials with each other or with other materials.
The Rock AR coating has become the basis for most visible AR coating designs. Depending on the index of the substrate and the index of the low-index material, there may be an optimum index for the halfwave layer to give the broadest low-reflection region. In a variation of the Rock AR coating, the halfwave third layer can be subdivided into two or more sublayers of various refractive indices to improve the width of the low-reflectance region. See Laird et al., "Durable Conductive Anti-reflection Coatings for Glass & Plastic Substrates," Soc. Vacuum Coaters, 39th Annual Technical Conference Proceedings, 361-365 (1996) and U.S. Pat. No. 4,128,303.
In the case of sputtered AR coatings, a preferred material from an optical point of view for the high-index third layer is TiO.sub.2, because of its high index of refraction. However, TiO.sub.2 has a slow deposition rate. The slow rate is only partially compensated for by running the TiO.sub.2 at a much higher power. Thus, a significant time and number of sputter cathodes must be devoted to the sputtering of the thick third layer. A further problem arises in the sputtering of temperature-sensitive materials, such as plastic film which cannot take temperatures over about 60.degree. C., or CRTs which cannot be exposed to temperatures above approximately 150.degree. C. In this case, the high power of the TiO.sub.2 sputter sources can easily overheat the substrates. To avoid over-heating, the TiO.sub.2 targets can be run at lower power, but only at the expense of a much slower coating process or one which requires many more TiO.sub.2 targets.
Several solutions to the problem of coating the high-index third layer in a more economical manner have been proposed. For example, the use of Nb.sub.2 O.sub.5 in place of TiO.sub.2, as disclosed in U.S. Pat. Nos. 5,372,874 and 5,450,238, has the advantage that the deposition rate of Nb.sub.2 O.sub.5 is about twice as fast as that of TiO.sub.2. Thus, the use of Nb.sub.2 O.sub.5 would decrease the time or number of cathodes required to deposit the high-index third layer by a factor of two. The disadvantage of using Nb.sub.2 O.sub.5 is that the cost of a niobium sputter target is about five times that of a similar titanium target. Similarly, other high-rate materials could be used for the high-index layers. One alternative is to divide the high-index halfwave layer into two quarterwave layers, one of TiO.sub.2 and the other of a high-rate material such as ZnO. See U.S. Pat. Nos. 5,105,310 and 5,270,858. While this provides an improvement in the overall cost of the AR coating, there is a drawback in terms of coating durability, since ZnO tends to be hydroscopic. In addition, one is still left with the quarterwave of TiO.sub.2.
Up until the present, all attempts to produce an economical broad-band AR coating by magnetron sputtering have been based on the Rock AR coating, which specifically has a high-index third layer or its equivalent which is approximately a halfwave thick. It is the object of the present invention to provide a broad-band AR coating in which the optical thickness of the high-index third layer is less than one quarterwave.
It is a further object of the present invention to provide an AR coating which is significantly more economical to produce by in-line magnetron sputtering than AR coatings based on the Rock AR coating design.